Odor monitoring of air emissions is not legally mandated in the United States. Rather, odor is regulated and monitored based on the number of complaints. Gas-phase biological reactors use microbial metabolic reactions to treat contaminated air. Biological treatment is effective and economical for low concentrations of contaminants in the air, especially when handling large quantities of air. The contaminants are sorbed from a gas to the water/biological fixed film, or suspended growth, where microbial attack occurs. Through oxidative or reductive reactions contaminants are converted to carbon dioxide (CO2), water vapor, and organic biomass. The most common gas-phase biological reactors are biofilters, biotrickling filters, and bioscrubbers. Devinny, J. S., Deshusses, M. A., Webster, T. S., Biofiltration for Air Pollution Control, Lewis Publishers, Boca Raton, Fla. (1999).
In the gas-phase biological reactors, an optimized balance of contaminated air, nutrients, oxygen, water and microbial population improves efficiency. Alonso, C., Modeling of VOC Degradation in Gas Streams, Ph.D. Thesis, University of Cincinnati, (1999). Alonso, C., et al., Mathematical Model for the Biodegradation of VOCs in Trickle Bed Biofilters, Water Science and Technology, 39, 7 (1999): 139–146. Alonso, C. et al., Mathematical Model and Parameter Estimation for Treatment of VOCs in Trickle Bed Biofilters, Proceedings of the 72nd WEF Annual Conference and Exposition, New Orleans, La. (1999). Alonso, C. et al., Dynamic Mathematical Model for the Biodegradation of VOCs in a Biofilter-Biomass Accumulation Study, Environmental Science and Technology, 32, 20 (1998): 3118–3123. Alonso, C. et al., Modeling of the Biodegradation Process in a Gas Phase Bioreactor-Estimation of Intrinsic Parameters, Proceedings of the 1998 USC-TRG Conference on Biofiltration, Los Angeles, Calif. (1998).
Biofilters generally pass humidified, contaminated air through a thick layer of peat moss or soil. Over time, this media compacts so that contaminated air/oxygen moves through a shortcut passage or crack, and only the microorganisms present in the passage are exposed to contaminated air. This “channeling effect” means that only a limited portion of the media is actually used. Although the media material is porous, air does not pass through the pores of compacted media.
Biofilters are not true filtration units but are systems that combine the basic processes of absorption, adsorption, desorption and degradation of gas phase contaminants. Typical biofilters employ microorganisms affixed to organic media such as compost or peat. Extensive study into the growth properties of microorganisms (e.g., bacteria) in recent years has shown that particular types of bacteria may exist in complex forms comprising layers that tenaciously adhere to surfaces. Upon adhering to a surface, these complex forms of bacteria are termed “biofilms.” Generally, biofilms are comprised of sessile bacteria, this particular type of bacteria contributing to their inherent tenacity. As the contaminated air passes through the organic media, the contaminants sorb onto the biofilm and are biodegraded by the microorganisms. Biofilters usually employ water to humidify the contaminated gas stream prior to entry into the biofilter and to add nutrients for the microorganisms. If humidification proves inadequate, direct irrigation of the bed may be employed. Over time, all conventional media compacts, necessitating replacement.
A biotrickling filter uses inorganic material, such as diatomaceous earth, ceramic, or glass beads, for its packed bed. A biological fixed film grows on this bed. Water is sprayed on top of the packed bed and contaminated air is fed counter-currently or co-currently. Biotrickling filters exhibit many of the phenomena of all biofilters. However, since a biotrickling filter hosts a thriving microbial population, excessive biomass growth and clogging are common problems.
In a bioscrubber, after initial contact with contaminants, absorption occurs. The contaminants are then degraded in a separate aeration tank. Absorption of contaminants may be achieved in a packed column, a spray tower, or a bubble column.
A gas-phase bioreactor is disclosed in U.S. Pat. No. 2,793,096, De-Odoring of Gas Streams by the Use of Micro-Biological Growths, to Pomeroy, May 21, 1957. This bioreactor uses soil beds to treat odorous sewer gases. In the last ten years, more stringent environmental requirements have renewed interest in gas-phase biological reactors. The following U.S. patents are directed to improving the efficiency of gas-phase biological reactors.
U.S. Pat. No. 4,999,302, Biological Contact Gas Scrubber for Waste Gas Purification, to Kahler et al., Mar. 12, 1991, rearranges a rotating biological contactor (RBC), a typical wastewater treatment unit, and feeds contaminated air into a series of chambers containing an RBC disc set. A defect of this design permits contaminated air to short-circuit through the space between the RBC and the housing. The air in the RBC disc set remains stagnant so that all available microorganisms are not used.
U.S. Pat. No. 5,413,936, Rotary Biofilter, to Rupert, May 9, 1995, rotates a horizontal cylindrical vessel filled with biofilter media. The purpose of rotation is to break up compacted media and to collapse any fissures. Although the rotation helps to reduce the compaction and destroy cracks, there is channeling in the media, the channeling becoming more apparent over time.
U.S. Pat. No. 5,714,379, Biodegradation of Volatile Organic Contaminants from Air Using Biologically Activated Foam, to Phipps, Feb. 3, 1998, employs biologically activated foam to treat contaminated air.
U.S. Pat. No. 5,766,938, Biological Deodorizing Apparatus with Rotary Carriers, to Hongo, Jun. 16, 1998, modifies the RBC system with a perforated high-density polyethylene disc and a water-scooping device.
U.S. Pat. No. 5,780,293, System and Method for Capturing and Destroying HAP/VOC Substances Using Microbial Degradation, to Seagle, Jul. 14, 1998, uses filtering media, such as activated carbon or zeolites, in a rotating drum. It passes contaminated air through the drum after it is scrubbed in a suspended growth solution. The scrubbed air escapes mainly through the space between the wall and drum and through cracks in the media.
The channeling effect is also a problem for the biotrickling filter. Zhu and others observed dense biomass growth in a biotrickling filter and had to backwash regularly to avoid clogging. When water is sprayed and clogging starts, water forms a channel in the biotrickling filter and contaminated air follows the channeling passages. Again, in the biotrickling filter, the microorganisms outside of the channel passage have a limited chance to contact contaminated air, oxygen, nutrients, and moisture. Zhu, Xueqing, A Fundamental Study of Biofiltration Process for VOC Removal from Waste Gas Stream, Ph.D. thesis, University of Cincinnati (2000). Zhu, X et al., The Influence of Liquid Flow Rates on VOC Removal in Trickle-Bed Biofilters, Proceedings of the AWMA Annual Meeting & Exhibition, St. Louis, Mo. (1999). Zhu, Xueqing et al., Biofilm Structure and Mass Transfer in a Gas Phase Trickle-bed Biofilter, 1st World Water Congress of the International Water Association, Water Science and Technology, Paris, France (July 2000). Zhu, X. et al., The Effect of Liquid Phase on VOC Removal in Trickle-Bed Biofilters, Water Science and Technology, 38, 3 (1998): 315–322.
Zhu and others found nitrate to be a better nitrogen source, but nitrate is a limiting factor when a highly biodegradable substance is treated. It was also found that gas-phase contaminated air can directly contact microorganisms without passing through the liquid layer. Zhu, X. et al., The Influence of Liquid Flow Rates on VOC Removal in Trickle-Bed Biofilters, Proceedings of the AWMA Annual Meeting & Exhibition, St. Louis, Mo. (1999). Rihn, M. J. et al., The Effect of Nitrate on VOC Removal in Trickle Bed Biofilters, Water Research, 31, 2997–3008 (1997). Zhu, X. et al., The Effect of Nitrate on VOC Removal in Trickle Bed Biofilters, Water Science and Technology, 34, 34 (1996): 573–581.
In order to overcome the nitrate-limiting condition, a gas-phase nitrogen source was suggested. In the bioscrubber, the chance of water drops meeting contaminated air is also limited. In order to increase the microorganisms' chance to contact contaminated air in a bioscrubber, Yu and others used a three-phase fluidized bed and found that suspended biomass and fixed film play different roles at different environmental conditions. Kim, Byung J. et al., Treatment of Volatile Organic Compounds from Gas Streams Using a Three-Phase Circulating-Bed Biofilm Reactor, ERDC/CERL TR-00-9, U.S. Army Corps of Engineers, Champaign, Ill. (2000). Yu, H. et al., Contributions of Biofilm Versus Suspended Bacteria in an Aerobic Circulating Bed Biofilm Reactor, 1st World Water Congress of the International Water Association, Water Science and Technology, Paris, France (July 2000). Yu, H. et al., Gas Phase Toluene Removal by Circulating Bed Biofilm Reactor, International Specialty Conference on Biofilm Processes, International Association on Water Quality, New York (November 1999). Yu, H. et al., Effects of Substrate and Oxygen Limitation on Gas-phase Toluene Removal in a Three-phase Biofilm Reactor, Water Science and Technology, (2001). As follow-on to the work of Yu, B. Sang et al. reported higher removal efficiency of smaller size carriers. Sang, B. et al., The Trade-Offs and Effect of Carrier Size and Oxygen Loading on Gaseous Toluene Removal—Performance of a Three-phase circulating Bed Biofilm Reactor, Applied Microbiology and Biotechnology (2003).
Dr. Byung Kim, the present inventor, used random-shaped engineered media (e.g., a flat square of polyurethane punched in the center with a circular hole and cut in half) to observe that contaminated air passed through spaces between the media. Little microorganism growth occurred inside of media pores, and only the surface of the media was actively used. Kim, Byung J. et al., Biofiltration of Solvent Vapors from Munitions Manufacturing Operations, CERL Technical Report 99/57, U.S. Army Corps of Engineers, Champaign, Ill. (1999).
Based on the above investigation results, Dr. Kim designed an improved closed biofilter, i.e., the first-generation rotating biofilter, for which design he obtained U.S. Pat. No. 6,403,366 B1, Method and Apparatus for Treating Volatile Organic Compounds, Odors, and Biodegradable Aerosol/Particulates in Air Emissions, to Kim, Jun. 11, 2002. C. Yang et al. compared single layer and multi-layered media using the above patent and reported higher efficiency of multi-layered media (i.e., with air gaps). C. Yang et al, Comparison of Single-layer and Multi-layer Rotating Drum Biofilters for VOC Removal, Environmental Progress, AIChE (2003). C. Yang et al also compared performances at different modes of operation. Yang, C. et al., Removal of Volatile Organic Compounds in a Hybrid Rotating drum Biofilter, Journal of Environmental Engineering, ASCE (2004). C. Yang, had also comprehensively evaluated the above patented biofilter at bench scale. Yang, C., Draft Ph.D. Thesis, University of Cincinnati (2004).
The above patent describes employment of a porous media of “microbial” foam that rotates in a closed reactor, improving the efficiency of gas-phase biological reactors by increasing the chances of the contaminants meeting with the oxygen, nutrients, and moisture needed for the microorganisms to work most effectively. In the first generation rotating biofilter, contaminated air is introduced to a stainless housing that contains the cylindrically shaped media system. Rotating the media with motor, chain, and sprocket is not straightforward, especially if the shaft diameter is equal to or greater than about 30 cm (1.0 ft.). Off-the-shelve chain and sprockets for such applications are not readily available commercially. In long-term operation, the sprocket and chain drive is the weakest point of the first generation rotating biofilter. Frequent replacement and repair is needed as experienced in operation of rotating biological contactors in wastewater treatment plants. Since the media is submerged in nutrient-enriched water for half of the time, there is much more water than the microorganisms require during the submerged cycle and at the beginning of the emerging cycle. Moreover, since the air stream flows from the circumference of the media to the center/shaft, airflow may force the biomass to move toward the center of the rotating biofilter.
Still, even the first-generation rotating biofilter reactor performs better than conventional gas-phase biological reactors because it overcomes channeling effects and uses the complete surface of all media pores as fully coated with a biological fixed film. The present invention improves upon the first-generation rotating biofilter to implement a second-generation open-bed biofilter with all of the benefits of the first-generation closed reactor and enhancements thereto.